1. Field of the Invention
The present invention relates to a self-sustaining container made of a saturated polyester resin, formed by biaxial stretch blow molding, which is suitable for filling e.g. a carbonated drink or a soft drink. More particularly, it relates to a self-sustaining container excellent in heat and pressure resistance during heat sterilization of the content.
2. Description of the Prior Art
Heretofore, as a heat and pressure resistant container, a container has been most common wherein the bottom is bulge-formed into a hemispherical shell shape to increase the internal pressure resistance of the main body of the container, and a base cup molded in the form of a closed-end cylinder, is attached thereto to impart a self-sustaining function to the container. However, use of such a base cup has various problems such that the base cup has to be separately molded, then attached and fixed to the container bottom, that the weight of the container increases, that the shape tends to be large-sized, that during the heat sterilization process, hot water does not adequately reach the bottom of the container, whereby heat sterilization of the content can not smoothly be carried out, and that in such heat sterilization, water tends to be trapped in the base cup and can not readily be removed.
From the viewpoint of conservation of resources or environmental protection, it is desired to reuse used empty containers. However, in the case of a container having a base cup attached thereto, the main body of the container and the base cup or the adhesive are made of different materials, and they must be separated for reuse, which adds to the cost of the recycling process.
In view of such problems, it has been desired to develop a heat and pressure resistant container which requires no base cup. For a pressure resistant container which requires no base cup, some proposals have been made, which are usually directed to either a champagne type structure or a structure in which a plurality of legs are radially bulged around the center of the bottom and valley lines are formed between the adjacent legs. Such structures are disclosed, for example, in Japanese Examined Patent Publications No. 5708/1973, No. 40693/1984 and No. 9170/1986 and Japanese Unexamined Patent Publications No. 202424/1988 and No. 43342/1991.
However, the containers disclosed in these publications do not provide adequate performance when they are used as heat and pressure resistant containers to be subjected to a heat sterilization process, although they may provide adequate performance as pressure resistant containers. Namely, the containers disclosed in these publications have such problems that when the temperature of the content rises to a level of from 50.degree. to 70.degree. C. during the heat sterilization, the internal pressure increases, and the material of the containers tends to undergo creeping, whereby the center of the bottom and the peripheral portion of the center of the bottom are likely to undergo creeping and project, whereby the container loses the self-sustaining stability.
As a method for solving this problem, it has been attempted to thermally crystallize the center of the bottom of the container. However, such a method is not desirable, since the peripheral edge of the thermally crystallized portion tends to have low strength and is likely to outwardly project by the internal pressure, and a deformation is likely to result, or stress cracking is likely to result.
The container disclosed in Japanese Unexamined Patent Publication No. 85535/1993 is the one obtained by preliminarily thermally crystallizing the center of the bottom of a preform to form a hemispherical shell-shaped bottom having a thermally crystallized bottom center and further subjecting this bottom to finish blow stretch molding by means of a mold to form legs. Such a container is the one wherein the resin remaining at the periphery of the thermally crystallized bottom center is adequately stretched and thin-walled. In this container, only the center of the bottom is crystallized, and the peripheral portion of the crystallized bottom center is thin-walled. Accordingly, when the internal pressure increases during heat sterilization, there will be a problem such that such a portion undergoes creeping, whereby the bottom will project to lose the self-sustaining stability. Or, even if the self-sustaining stability is maintained, the content level drops substantially to lose practical usefulness.
As a result of an extensive research, the present inventors have surprisingly found that in the case of a bottom structure of a biaxial stretch blow-molded self-sustaining container, wherein a plurality of legs are radially bulged around the center of the bottom and valley lines are formed between the adjacent legs, the stress by the internal pressure is concentrated especially at the peripheral portion of the center of the bottom and at the valley lines and further that in the projection of the bottom at the time of heat sterilization, creeping is particularly remarkable at the portion of each valley line close to the center.